Thanks to
a200337 (git-am: propagate -C<n>, -p<n> options as well,
2008-12-04) and commits around it, "git am" is equipped to correctly
propagate the command line flags such as -C/-p/-whitespace across a patch
failure and restart.
It is trivial to support --directory option now, resurrecting previous
attempts by Kevin and Simon.
Signed-off-by: Junio C Hamano <gitster@pobox.com>